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The Nekton Assemblage of Salt Marsh Pools in a Southeastern United
States Estuary
Marvin M. Mace III
1
&Matthew E. Kimball
1
&Eric R. Haffey
1
Received: 15 March 2018 /Revised: 8 August 2018 /Accepted: 9 August 2018 /Published online: 27 August 2018
#Coastal and Estuarine Research Federation 2018
Abstract
Marsh pools are present in estuaries throughout the world and provide valuable habitat for fishes and decapod crustaceans (i.e.,
nekton). The purpose of our study was to examine the species composition and temporal variation of the nekton assemblage
within marsh pools of a southeastern US estuary. We conducted weekly sampling of five marsh pools in the North Inlet estuary,
SC from May to November 2016. Temporal variation in the nekton assemblage appeared to be related to the life history of
individual species, tidal connectivity of pools with adjacent habitats, and environmental conditions within pools. Most transient
species, which migrate into the North Inlet estuary as larvae or juveniles, were present primarily in early summer and late fall.
Many transient species were absent or occurred in low abundance during July and August when water temperature was highest,
salinity most variable, and tidal connectivity with adjacent habitats was lowest. In contrast, most resident species, which can
complete their entire life cycle within the North Inlet estuary, were present and relatively abundant throughout the study as
juveniles and adults. Based on the limited studies available, species richness and the ratio of transient to resident species in marsh
pools at low latitudes (e.g., southeastern US) are higher compared to marsh pools at high latitudes (e.g., east coast of Canada). A
more comprehensive understanding of the role of marsh pools in the life history of nekton would be useful for conserving,
managing, and restoring salt marshes and the species found in these environments.
Keywords Estuary .Marsh pools .Nekton .Resident .Salt marsh .Transient
Introduction
Marsh pools occur in estuaries throughout the world and are
one component within the mosaic of interconnected salt marsh
habitats (Minello et al. 2003). Defining characteristics of
marsh pools, suchas surface area, depth, and connections with
adjacent aquatic habitats, can vary greatly. Herein, we focus
on high marsh pools that are generally shallow (< 0.5 m),
small (surface area = 10s to 100s m
2
), soft-bottomed depres-
sions embedded in the marsh surface that hold water through-
out the tidal cycle and have no permanent connection (i.e.,
channel) to adjacent water bodies (Harshberger 1916;Nicol
1935;Ingólfsson1994; Rowe and Dunson 1995; Adamowicz
and Roman 2005; MacKenzie and Dionne 2008; Davis et al.
2014a). Pools are only intermittently linked to other aquatic
habitats in the salt marsh, such as tidal creeks, primarily during
spring high tides and storm events when the marsh surface is
flooded. Local weather conditions (e.g., air temperature and
precipitation) and the frequency of tidal inundation influences
environmental conditions within marsh pools (Nicol 1935;
Noël and Chmura 2011). Compared to nearby tidal creeks,
environmental conditions within pools can be extreme
(Hunter et al. 2007); wide ranges in water temperature (−1.8
to 41 °C), salinity (0 to 60), and dissolved oxygen (0 to
20 ppm) levels are possible (Rowe and Dunson 1995;
Layman et al. 2000;Raposa2003; Smith and Able 2003).
The fish and decapod crustacean (i.e., nekton) communities
of marsh pools have been examined primarily in estuaries
along the east coast of North America (see Able et al. 2005
and references therein) and to a lesser extent in Europe (Nicol
1935; Verhoeven and van Vierssen 1978;Frid1988;Fridand
James 1989;Ingólfsson1994;Hampeletal.2004), Australia
(Davis et al. 2012,2014a,b), South America (Sampaio and
Martinelli-Lemos 2014), and the west coast of North America
(Wolf et al. 1983; Barnby et al. 1985). Most of these studies
Communicated by Charles T. Roman
*Marvin M. Mace, III
marvin.mace.iii@gmail.com
1
Baruch Marine Field Laboratory, University of South Carolina, PO
Box 1630, Georgetown, SC 29442, USA
Estuaries and Coasts (2019) 42:264–273
https://doi.org/10.1007/s12237-018-0450-3
are limited to describing the presence and species richness of
nekton within marsh pools; however, detailed studies along
the northeastern coast of North America (i.e., north of Cape
Hatteras, North Carolina) have found the species richness of
nekton to be relatively low compared to adjacent marsh hab-
itats (Rowe and Dunson 1995; Layman et al. 2000;Raposa
and Roman 2001;Ableetal.2005) and that pools may func-
tion as locations for nekton to overwinter, forage, and repro-
duce (Chidester 1920; Bleakney and Meyer 1979; Worgan
and FitzGerald 1981a,b; Ward and FitzGerald 1983; Talbot
and Able 1984; Walsh and FitzGerald 1984;Talbotetal.
1986; Poulin and FitzGerald 1989; Whoriskey and
FitzGerald 1989; Smith and Able 1994; Rowe and Dunson
1995; Halpin 2000;Laymanetal.2000;RaposaandRoman
2001;Raposa2003;Ableetal.2005,2012; MacKenzie and
Dionne 2008;Hunteretal.2007,2009; Vincent et al. 2015).
Although marsh pools occur in estuaries along the east
coast of North America at lower latitudes (i.e., south of
Cape Hatteras, North Carolina), considerably less infor-
mation is available on nekton use of pools in this region.
The presence and species richness of nekton in marsh
poolsinthisregionhavebeenexaminedinfewlocations
(Kilby 1955; Harrington and Harrington 1961;Dahlberg
1972; Subrahmanyam and Coultas 1980), and the function
(e.g., feeding, reproduction) of marsh pools has been
identified for a few species (Rickards 1966,1968;Kneib
1978,1982). From these limited studies, it appears that
marsh pools in the southeastern US may provide nursery
habitat and offer similar benefits to nekton as has been
observed for pools elsewhere. Further, marsh pools in this
region may support more species rich nekton assemblages
than pools at higher latitudes along the east coast of North
America. However, general information on nekton use of
marsh pools, as well as information on the biological and
physical factors that may cause variation in the presence
and composition of nekton assemblages, is lacking for
most of this region.
The purpose of our study was to examine the species
composition and temporal variation of the nekton assem-
blage within marsh pools of a southeastern US estuary.
We conducted weekly sampling over a 28-week period
in five marsh pools within the North Inlet estuary, SC,
USA. Our sampling targeted multiple life history stages
of nekton to determine if marsh pool use varied among
species and life history stage. During this same time pe-
riod, we measured environmental variables within pools,
such as salinity and temperature, and pool connectivity
with adjacent habitats to identify abiotic factors that could
potentially influence nekton use of marsh pools. We also
examined studies from marsh pools in eastern North
America and worldwide to identify potential spatial vari-
ation in nekton utilization of these habitats in the region
and across the globe.
Materials and Methods
The nekton assemblage in five marsh pools (as defined above)
within a 0.1 km
2
area in the central portion of the North Inlet
estuary was sampled weekly during 6 May to 18 November
2016 (n= 28 weeks). Pools were shallow (< 0.5 m at low tide),
relatively small (surface area ranged from 70 to 568 m
2
;
Tab le 1), and surrounded by either short Spartina alterniflora
high marsh or Salicornia–Distichlis marsh described by Teal
(1958). All pools were sampled on the same day during low
tide when the surrounding marsh was not inundated with wa-
ter, and pools were not connected to any adjacent aquatic
habitats. We chose to sample pool nekton from spring through
fall because this is typically the period of highest abundance
for fish and decapod crustacean species within the North Inlet
estuary (Allen et al. 2014).
Larvae and other small nekton were targeted using a cus-
tom Sea Gear rectangular plankton net (20-cm high × 1.4-m
wide with 1-mm mesh), which allowed us to sample most of
the water column in the shallow pools. The plankton net was
pulled once along a fixed transect along the pool’s longest axis
for each sampling event. These samples were immediately
placed on ice and transported to the laboratory for processing
(approximately 2 h after collection). All individuals were
identified to the lowest feasible taxonomic level, counted,
and up to 20 individuals of each taxon were randomly selected
and measured for length (mm): standard length (SL) for fishes,
carapace width (CW) for crabs, and total length (TL; tip of
rostrum to tip of telson) for shrimps.
After the plankton net sample was collected at each pool, a
cast net (1.8-m diameter with 6-mm mesh) was used to target
large nekton with three replicate casts thrown from the pool
edge. Within the same pool, cast net samples were collected
approximately 5 to 10 min after completion of the plankton
sample and replicate cast net samples were separated by ap-
proximately 2 to 5 min. Cast net samples were sorted in the
field where all individuals were identified, counted, and up to
20 individuals of each taxon were randomly selected and mea-
sured as described above. All individuals were then released
back into the pools except for individuals identified as
Megalops atlanticus, which were placed on ice and
transported to the laboratory for use in other studies.
Taxon-specific density was calculated for taxa identified
from the plankton net samples using the volume of water
filtered per sample (height × width of plankton net × length
of tow) and standardized to number of individuals per
100 m
3
. A mean density for each sample date was calculated
by averaging densities from all five pools. The number of
individuals of a given taxon was determined for each cast
net sample (i.e., catch-per-unit-effort; CPUE), then a mean
CPUE was calculated for each pool on each sample date.
Mean CPUE values for each pool on each sample date were
then used to calculate a mean CPUE for each sample date and
Estuaries and Coasts (2019) 42:264–273 265
also a mean CPUE ± standard error over the entire study
period.
Environmental conditions within marsh pools were moni-
tored both discretely, during nekton sampling events, and con-
tinuously over the entire study period. Prior to collecting sam-
ples from each pool on each sample date, water salinity and
dissolved oxygen (DO) concentration were measured with a
handheld multiparameter meter (YSI, Inc.). Water depth and
temperature were monitored continuously at 15-min intervals
throughout the study period using a HOBO water level and
temperature logger (U20L-01; Onset Computer Corp.) located
in the deepest portion of each pool. Water depth measure-
ments taken using a meter stick at the edgeof each pool during
high tide were used in conjunction with simultaneous data
from the HOBO logger to determine a depth reading on the
HOBO logger at which the water depth was 5 cm in the sur-
rounding marsh. At this stage of inundation, pools would be
connected by a water depth (≥5 cm) that we assumed would
be sufficient to allow nekton to move between the marsh pool
and nearest subtidal creek. Pool connectivity was tracked by
creating an index of daily hydrological connectivity, which
was calculated for each pool as the percent of daily 15-min
intervals (n= 96 intervals per 24 h) when the water depth
reading on the HOBO was at a level where water depth on
the surrounding marsh was at least 5 cm above the edge of the
pool. This resulted in 197 percentage values for each pool (1 per
day × 197 days). We then took the mean of these percentage
values on each day among all five pools to examine the hydro-
logical connectivity of the pools over the entire study period.
Descriptive statistics, such as mean, maximum, and mini-
mum values, were used to examine nekton and environmental
data. Our primary objective was to describe patterns in nekton
occurrence and environmental factors that we observed; there-
fore, we did not test any statistical hypotheses. Nekton taxa
were assigned an estuarine status based on whether their entire
life cycle (resident) or only a portion (transient) occurred with-
in the estuary. Taxon-specific patterns in marsh pool utiliza-
tion were examined by plotting mean CPUE and density over
time separately for cast net and plankton net sample data,
respectively. Before plotting, mean CPUE and density data
were scaled by the maximum value observed during the study,
so the trend over time can be easily compared among different
taxa even if the absolute values vary among taxa. Length-
frequency distributions were also examined for nekton taxa
for which at least 50 individuals were collected from both sam-
pling gears combined. Larvae collected for a given taxon were
examined separately from juveniles. Environmental data were
examined by plotting mean, minimum, and maximum values
over time for discretely and continuously collected data.
Results
A total of 13,478 individuals distributed among 26 taxa were
collected during May through November 2016 (Table 2).
Most taxa were fishes (n= 21), but five decapod crustaceans
(Acetes americanus,Callinectes sapidus,Farfantepenaeus
spp., Litopenaeus setiferus,andPalaemonetes spp.) were also
collected. The two most abundant taxa, Palaemonetes spp.
and Cyprinodon variegatus, accounted for 75% of all individ-
uals collected. Of the 26 taxa collected, nine were estuarine
residents, comprising 88% of all individuals, while 17 were
transients and made up 12% of the total catch. Length among
all individuals ranged from 4 to 154 mm, but most (99.7%)
were < 100 mm. Only two species, M. atlanticus and E.
saurus, were collected as larvae.
Temporal patterns in CPUE and density for the most abun-
dant taxa (n=16;≥50 individuals total collected; 99% of the
total catch) were generally related to whether the taxa were an
estuarine resident or transient (Fig. 1). Many of the most abun-
dant transient taxa (n= 10) were absent or occurred in rela-
tively low abundance during mid-summer (July and August);
however, the most abundant resident taxa (n=6)werepresent
and relatively abundant throughout the study period, but there
were exceptions (Fig. 1). C. sapidus was relatively abundant
in summer, and E. saurus (larvae and juveniles) and M.
curema were present in low numbers throughout the study
period. M. atlanticus (larvae and juveniles) was only present
from June through October. In contrast, Palaemonetes spp.
was only present during September through November.
Length-frequency distribution patterns differed between
residents and transients and by gear type (Fig. 2). While the
length of resident and transient taxa overlapped; as expected,
transient taxa were generally larger than resident taxa. Within
taxa, some overlap was observed in the length of individuals
between sampling gears, but for transient species in general,
individuals collected with the cast net tended to be larger (on
average) than individuals collected with the plankton net. For
Table 1 Location (latitude and
longitude), surface area (m
2
), and
distance to the nearest subtidal
creek (m) of five marsh pools in
the North Inlet estuary
Pool Latitude Longitude Surface area Distance to nearest creek
1 332003.76 −79 11 44.36 427 m
2
307 m
2 332004.89 −79 11 45.58 70 m
2
330 m
3 331956.50 −79 11 53.50 368 m
2
204 m
4 331956.90 −79 11 54.09 115 m
2
296 m
5 331942.79 −79 12 12.58 568 m
2
276 m
266 Estuaries and Coasts (2019) 42:264–273
example, the mean length ± standard deviation (SD) of L.
setiferus collected with the plankton net was 33 ± 9-mm TL,
while the mean length ± SD of L. setiferus collected with the
cast net was 57 ± 18-mm TL. In contrast, the size of resident
species individuals collected with the two gears was
generally similar (Fig. 2). In general, based on the length-
frequency distributions, most resident species were represent-
ed by both juvenile and adults, while transient species were
represented primarily by juveniles (only two transient species
were collected as larvae and small juveniles), although some
transient individuals may have been or were likely mature
individuals, especially the largest C. sapidus.
Long- and short-term patterns in environmental vari-
ables were observed within the pools. Mean water salinity
ranged from 14 to 42 and was most variable among sam-
pling dates during July and August (Fig. 3). No clear
pattern in discrete DO averages was observed from May
through November; however, a daily pattern was evident
with values generally lowest at 09:00 (approximately
4 mg/L on average) and highest at 17:00 (approximately
11 mg/L on average). Daily mean water temperature
ranged from 13 to 33 °C, and the pattern in water tem-
perature during May to November was typical for this
region; highest temperatures occurred during July and
Table 2 Nekton taxa collected from five marsh pools in the North Inlet
estuary from May through November 2016 using a cast net (1.8-m
diameter with 6-mm mesh) and plankton net (20-cm high× 1.4-m wide
with 1-mm mesh). Taxa are presented in alphabetical order. Estuarine
status refers to the classification of a taxon as an estuarine resident (R)
or transient (T). The total number of individuals collected (N
total
)and
measured (N
measure
), along with the mean (± one standard error) and max-
imum catch-per-unit-effort (CPUE; individuals/net) and density (individ-
uals/100 m
3
), is reported for cast net and plankton net catches,
respectively
Cast Net Plankton Net
CPUE Density
Species Estuarine Status N
Tota l
N
Measure
Mean ± SE Maximum N
Total
N
Measure
Mean ± SE Maximum
Acetes americanus T 0 - - - 1 1 0.13 ± 0.13 3.5
Anchoa mitchilli T 14 14 0.03 ± 0.02 0.27 0 - - -
Brevoortia tyrannus T 12 12 0.03 ± 0.03 0.73 0 - - -
Callinectes sapidus T 136 136 0.32 ± 0.04 0.93 25 25 2.7 ± 0.68 13.85
Cyprinodon variegatus R 2546 2059 6.06 ± 0.7 17.47 1945 903 264.89 ± 65.95 1721.17
Elops saurus T 28 28 0.07 ± 0.02 0.4 117 116 16.57 ± 5 205.48
Elops saurus (larvae) T2 2 0 ± 0 0.07 113 111 15.02 ± 3.58 122.85
Eucinostomus spp. T29 29 0.07 ± 0.03 1 31 30 3.77 ± 1.4 51.27
Farfantepenaeus spp. T79 79 0.19 ± 0.06 1.33 48 48 5.21 ± 1.42 32.34
Fundulus heteroclitus R 292 291 0.7 ± 0.09 1.8 591 411 89.45 ± 21.93 771
Fundulus luciae R 0 - - - 4 4 0.66 ± 0.33 10.45
Fundulus majalis R 311 243 0.74 ± 0.26 6.13 202 180 30.19 ± 6.63 186.88
Gambusia holbrooki R 17 17 0.04 ± 0.02 0.4 134 134 15.92 ± 3.44 67.6
Gobionellus oceanicus T 11 0±0 0.07 0- - -
Gobiosoma spp. T0 - - - 1 1 0.13 ± 0.13 3.5
Lagodon rhomboides T 5 5 0.01 ± 0.01 0.27 0 - - -
Leiostomus xanthurus T 71 71 0.17 ± 0.09 2.4 7 7 0.65 ± 0.56 15.53
Litopenaeus setiferus T 151 151 0.36 ± 0.1 2.87 15 15 1.93 ± 1.3 40.82
Lucania parva R 0 - - - 1 1 0.09 ± 0.09 2.59
Megalops atlanticus T 162 161 0.39 ± 0.09 2.33 88 88 13.89 ± 5.18 193.18
Megalops atlanticus (larvae) T0 - 36 36 4.82 ± 1.34 54.4
Menidia beryllina R 6 6 0.01 ± 0.01 0.2 30 30 3.95 ± 1.51 28.01
Menidia spp. T0 - - - 8 8 1.2 ± 0.91 33.7
Mugil cephalus T 203 199 0.48 ± 0.12 2.87 28 28 4.17 ± 1.37 45.99
Mugil curema T 146 146 0.35 ± 0.06 0.93 14 13 2.43 ± 1.17 23.23
Mugil spp. T50 45 0.12 ± 0.12 3.33 0 - - -
Palaemonetes spp. R3964 967 9.44 ± 2.73 94.4 1674 398 273.1 ± 100.47 2686.72
Poecilia latipinna R 87 89 0.21 ± 0.05 1 53 53 8.19 ± 2.95 98.64
Estuaries and Coasts (2019) 42:264–273 267
Relative CPUE / Density
Callinectes sapidus (T)
0.5
1
Elops saurus (T)
0.5
1
Elops saurus (larvae)(T)
0.5
1
Eucinostomus spp. (T)
0.5
1
Farfantepenaeus spp. (T)
0.5
1
Leiostomus xanthurus (T)
0.5
1
Litopenaeus setiferus (T)
Cast Net
Plankton Net
Megalops atlanticus (T)
Megalops atlanticus (larvae)(T)
Mugil cephalus (T)
Mugil curema (T)
Mugil spp. (T)
Cyprinodon variegatus (R)
Fundulus heteroclitus (R)
Fundulus majalis (R)
Gambusia holbrooki (R)
Palaemonetes spp. (R)
Poecilia latipinna (R)
Ma
y
Jun Jul Au
g
Sep Oct Nov Ma
y
Jun Jul Au
g
Sep Oct Nov Ma
y
Jun Jul Au
g
Sep Oct Nov
Fig. 1 Relative CPUE and density for the most abundant nekton taxa
(n= 16) collected with cast nets (catch-per-unit-effort, CPUE;
individuals/net) and plankton nets (density; individuals/100 m
3
) from
five marsh pools during May to November 2016 in the North Inlet
estuary. Mean CPUE and density on a given sample date are relative to
the maximum CPUE and density observed during the study period
(reported in Table 2). The estuarine status of each taxa is reported in
parentheses as either an estuarine resident (R) or transient (T). Taxa with
at least 50 individuals collected from both gears combined are presented
(totals reported in Table 2)
Relative Frequency
Callinectes sapidus (T)
0
0.5
Elops saurus (T)
0
0.5
Elops saurus (larvae)(T)
0
0.5
1
Eucinostomus spp. (T)
0
0.5
1
Farfantepenaeus spp. (T)
0
0.5
Leiostomus xanthurus (T)
0
0.5
Litopenaeus setiferus (T)
0
0.5
Megalops atlanticus (T)
0
0.5
Megalops atlanticus (larvae)(T)
0
0.5
1
Mugil cephalus (T)
0
0.5
Mugil curema (T)
0
0.5
1
Mugil spp. (T)
0
0.5
1
Cyprinodon variegatus (R)
0
0.5
Cast Net
Plankton Net
Fundulus heteroclitus (R)
0
0.5
Fundulus majalis (R)
0
0.5
Gambusia holbrooki (R)
0
0.5
Palaemonetes spp. (R)
0
0.5
1
Poecilia latipinna (R)
0
0.5
0 25 50 75 100 125 150 0 25 50 75 100 125 150 0 25 50 75 100 125 150
Size (mm)
Fig. 2 Relative length-frequency distributions of the most abundant nek-
ton taxa (n= 16) collected with a cast net (solid bars) and plankton net
(open bars) from five marsh pools during May to November 2016 in the
North Inlet estuary. The number of individuals in each length class is
relative to the total number of individuals measured (reported in
Table 2). Taxa with at least 50 individuals collected from both gears
combined are presented (totals reported in Table 2)
268 Estuaries and Coasts (2019) 42:264–273
August (Fig. 3). Mean daily water depth varied from 0.20
to 0.72 m, but on most (95%) days during May through
November, daily mean water depth was between 0.22 and
0.37 m. Marsh pools were hydrologically connected to the
nearest subtidal creek (see Table 1) for some amount of
time on 167 days during the 197-day study period (85%
of the days). Mean percent daily hydrological connectiv-
ity was highest during May to June and September to
November and lowest during July and August (Fig. 3).
During July and August (n= 62 days), the mean percent
daily hydrological connectivity was zero (i.e., pools were
not connected to nearby subtidal habitats) for 34% of the
days. In contrast, during May through June (n=56 days)
and September through November (n= 79 days), the mean
percent daily hydrological connectivity was zero for only
5 and 8% of the days, respectively. The peak in water
depth and connectivity during October was due to
Hurricane Matthew, which made landfall in SC on
October 8, 2016 approximately 35 km south of the
North Inlet estuary (Fig. 3).
Discussion
A variety of estuarine resident and transient nekton, including
larvae, juveniles, and adults, were collected from marsh pools
in the North Inlet estuary from May through November.
Nekton species richness in these pools (n=26taxa)waswith-
in the range reported from other marsh pools throughout the
southeastern US (Fig. 4, < 35° N latitude) and generally higher
than nekton species richness in pools along the northeastern
US and Canada Atlantic coasts (Fig. 4, > 35° N latitude).
Another difference in nekton species assemblages among
marsh pools in eastern North America is a general decrease
in the ratio of transient to resident species occurring in marsh
pools with increased latitude (Fig. 4). Many of the transient
species that occur within marsh pools at lower latitudes in-
clude the early life stages of important recreational (e.g., E.
saurus,M. atlanticus,Centropomus undecimalis) and com-
mercial fishery (e.g., penaeid shrimps, C. sapidus) species
(Kilby 1955; Harrington and Harrington 1961;Rickards
1968; Dahlberg 1972; Subrahmanyam and Coultas 1980).
0
10
20
30
40
50
Salinity
0
5
10
15
DO (mg/L)
0
10
20
30
40
50
Temperature (°C)
0
0.5
1
1.5
2
Depth (m)
Ma
y
Jun Jul Au
g
Se
p
Oct Nov De
c
0
25
50
75
100
Percent
Daily Hydrological
Connectivity
Fig. 3 Mean salinity, dissolved oxygen (DO, mg/L), temperature (°C),
water depth (m), and percent daily hydrological connectivity observed
within five marsh pools during May through November 2016 in the North
Inlet estuary. Salinity and DO were measured during weekly sampling
with a handheld YSI. Water temperature and depth were recorded every
15 min using a HOBO water level and temperature logger (U20L-01)
placed in each pool. Dotted lines represent the minimum and maximum
values for each 15-min interval among all five pools. Mean percent daily
hydrological connectivity represents the percentage of 15-min intervals
within a given day (n= 96 intervals per 24 h) when water depth was at
least 5 cm on the marsh surface surrounding each pool, at which point the
pool was considered hydrologically connected to the nearest tidal creek
Estuaries and Coasts (2019) 42:264–273 269
Despite differences in nekton species composition, some spe-
cies occur in pools along much of the coast, such as F.
heteroclitus (Georgia to Nova Scotia; Dahlberg 1972;
Bleakney and Meyer 1979)andC. variegatus (Florida to
Rhode Island; Harrington and Harrington 1961; Roman et
al. 2002). Based on length-frequency distributions, it appears
that both juveniles and adults of estuarine residents are present
in pools, while it is primarily the juvenile life history stage of
estuarine transients that were collected in pools. One notable
exception to this was the larval E. saurus and M. atlanticus
that appear to recruit directly to these habitats. Larvae of es-
tuarine resident species were likely present in habitats directly
adjacent to the marsh pools (e.g., shallow microhabitats on the
marsh surface; Kneib 1997), although our sampling effort did
not cover these habitats. Along with geographic location, dif-
ferences in nekton species richness in marsh pools could be
due to variation in pool size, depth, distance from the nearest
subtidal habitat, and location along the marsh elevation gradi-
ent (e.g., low to high marsh); however, where possible, we
sought to limit our comparisons to marsh pools as defined in
this paper.
Tidal flooding of the marsh surface ultimately controls the
connectivity of pools with nearby subtidal creeks; therefore,
spatial and temporal variation in tidal regime governs nekton
access to marsh pools (Rozas 1995). All of the marsh pools
included in our study were relatively similar in their elevation
along the intertidal gradient, but we observed differences in
the patterns of nekton CPUE or density over time that ap-
peared to be related to tidal connectivity. During July and
August, when pools and nearby creeks were infrequently hy-
drologically connected, the relative abundance (density or
CPUE) of many estuarine transient taxa was low or zero.
These transient taxa were primarily larvae and juveniles of
marine species that migrated into the North Inlet estuary. In
contrast, most resident species were present throughout the
summer period. Tidal connectivity is important for transient
species because a hydrological connection to adjacent habitats
allow these species to emigrate from pools to complete their
life cycle and avoid potentially lethal conditions, such as low
temperatures during winter (Mace et al. 2017). The effect of
tidal connectivity on individual organisms over short time
scales (e.g., consecutive tidal cycles) has rarely been exam-
ined, and therefore, residence times for individuals of resident
or transient species using marsh pools are not well understood
(but see Hunter et al. 2009; Able et al. 2012 for Fundulus
heteroclitus). Future studies using mark-recapture techniques
would be useful in determining residence time and site fidelity
for taxa in marsh pools and how these metrics vary among
estuarine resident and transient species.
Environmental conditions, such as water temperature, sa-
linity, and DO concentration, may influence marsh pool nek-
ton by affecting the growth and survival of individuals present
in marsh pools. We observed a large range for all environmen-
tal variables and a relatively large degree of variation in envi-
ronmental variables over time. For example, DO varied from
1.04 to 13.45 mg/L and tended to be lowest in the morning
and highest in the afternoon, which is similar to diel patterns
observed in other locations (Nicol 1935; Rowe and Dunson
1995; Smith and Able 2003). Salinity also varied greatly (14
to 42) within marsh pools over time, as reported by others
(Nicol 1935; Smith and Able 2003;NoëlandChmura
2011). Environmental variables, such as temperature and sa-
linity, may also interact with hydrological connectivity (Nicol
1935). In our study, variation in salinity was highest during
July and August when hydrological connectivity was lowest,
which was likely due to combinations of an increase in evap-
oration, variation in precipitation, and infrequent tidal
flooding during this period. Relatively extreme environmental
conditions within pools likely limit the number and relative
abundance of nekton species present in marsh pools. Species
capable of tolerating a wide range of environmental condi-
tions, such as resident cyprinodontoid fishes (Nordlie 2006),
and species that have adapted to deal with potentially lethal
conditions (e.g., low DO) via specialized behaviors (e.g., M.
Species Richness
25 30 35 40 45 50
0
10
20
30
40
50
1
2a
2b
3
4
5
6
789
10 11
Ratio Transient : Resident
Species
25 30 35 40 45 50
0
2
4
6
1
2a
2b 3
4
56
78
9
10 11
Latitude (°N)
Fig. 4 Species richness and the ratio of transient to resident species
reported from published studies on marsh pool nekton assemblages in
eastern North America. Numbers represent an individual study and are
presented in order of increasing latitude (south to north): 1. Harrington
and Harrington (1961), 2a-b. Kilby (1955), 3. Subrahmanyam and
Coultas (1980), 4. Dahlberg (1972), 5. Rickards (1968), 6. present study,
7. Rowe and Dunson (1995), 8. Able et al. (2005), 9. Raposa and Roman
(2001), 10. Bleakney and Meyer (1979), and 11. Worgan and FitzGerald
(1981b). We treated the two locations from Kilby (1955), Bayport (2a)
and Cedar Key (2b), as separate data points because they are separated by
approximately 1° of latitude, represented different environmental condi-
tions, and had different nekton assemblages
270 Estuaries and Coasts (2019) 42:264–273
atlanticus breathing air at the water’s surface) are better able to
withstand large fluctuations in environmental conditions that
are likely to occur in pool habitats.
Factors other than those we directly examined in our study
could also affect the presence and abundance of nekton in
marsh pools. Competition for food resources among and with-
in species could affect nekton presence, abundance, and
growth in marsh pools (Layman et al. 2000); especially during
periods when connectivity is low and individuals are not able
to migrate from pools. Predation may also affect nekton within
marsh pools (Kneib 1982). For example, juvenile M.
atlanticus are known to feed on estuarine resident fishes and
crustaceans (Rickards 1966), and C. variegatus has been ob-
served in the stomach contents of M. atlanticus collected from
North Inlet estuary pools (M. Mace, unpublished data).
During September, when M. atlanticus abundance was
highest, C. variegatus abundance was lowest, possibly
reflecting the impact of predation by M. atlanticus. Future
studies involving manipulative experiments and examination
of stomach contents could help to elucidate the effect of bio-
logical interactions among nekton on observed nekton assem-
blage patterns.
Our examination of other studies on marsh pool use by
nekton globally revealed several interesting patterns. Nekton
species richness is low in marsh pools at high latitudes. This
pattern was consistent for pools within estuaries of Europe
(Nicol 1935;Ingólfsson1994) and the west coast of North
America (Wolf et al. 1983), which is similar to the US and
Canada Atlantic coasts (Fig. 4). Pools in estuaries at low lat-
itudes, such as those in North America (Rickards 1968;
Subrahmanyam and Coultas 1980) and Australia (Davis et
al. 2012), appear to support more speciose and diverse nekton
communities, however, relatively few studies have been con-
ducted in this zone. Similar families of nekton were also found
utilizing marsh pools from different areas around the world.
Fishes from the family Gasterosteidae (sticklebacks) reside in
marsh pools along the east and west coasts of North America
and in Europe (Verhoeven and van Vierssen 1978; Worgan
and FitzGerald 1981a,b;Wolfetal.1983;Barnbyetal.
1985; Ingólfsson 1994), and shrimp from the family
Palaemonidae are present in marsh pools of Europe and the
east coast of North America (present study; Rickards 1968;
Verhoeven and van Vierssen 1978;Frid1988;Hampelet
al. 2004). In addition, larvae and juveniles of economical-
ly important fishes (present study, Rickards 1968;Wolfet
al. 1983;Frid1988; Davis et al. 2012), shrimps (present
study; Rickards 1968;F
rid1988;Hampeletal.2004;
Sampaio and Martinelli-Lemos 2014), and crabs (present
study; Rickards 1968; Subrahmanyam and Coultas 1980)
occurinmarshpoolsinmanyregionsaroundtheglobe;
this finding suggests that marsh pools may provide valu-
able nursery habitat for estuarine transient species
worldwide.
The extent of marsh pools in southeastern US estuaries and
their function in the life history of nekton is not well known.
Therefore, it is difficult to assess the overall value of habitat
provided by high marsh pools on nekton populations in this
region. In contrast, their extent has been well documented in
some regions (e.g., New England; Adamowicz and Roman
2005), where these pools are a common feature of the marsh
landscape and are important habitats for critical life history
functions of fishes (e.g., Able et al. 2005) and also serve as
foraging habitats for numerous waterbirds (Erwin et al. 2006).
Although our results are based on one location in one year,
when compared with observations from earlier studies fo-
cused on southeastern US estuaries our results support the
conclusion that marsh pools appear to serve as valuable hab-
itat for common resident and transient salt marsh nekton spe-
cies in this region. For example, E. saurus,M. atlanticus,and
C. sapidus use marsh pools extensively during their larval and
juvenile stages as do juvenile and adult stages of resident
nekton. It is unclear, however, whether or how individuals of
these transient species contribute to adult populations. A more
comprehensive understanding of the function of marsh pools
for nekton, their function relative to other habitats in the estu-
arine ecosystem, and how these functions vary geographically
would be useful for supporting efforts to conserve, manage,
and restore salt marshes and the species that depend on these
systems.
Acknowledgments We thankA. Adams and J. Wilson from the Bonefish
& Tarpon Trust for their suggestions and guidance with the development
and execution of this project. We would also like to thank faculty, staff,
and students from the USC Baruch MarineField Laboratory (D. Allen, S.
Forehand, M. Kennedy, P. Kenny, T. Thomas), Cornell College Rogers
Fellowship in Environmental Studies program (R. Bulger, J. Dean, J.
Tesensky), and Wofford College (K. Dickson, D. Kusher, K.
Moorhouse) for their assistance with this study. The suggestions of L.
Rozas, the associate editor, and two anonymous reviewers improved the
manuscript. This research was conducted in accordance with the guide-
lines set forth in University of South Carolina IACUC Animal Care and
Use Protocols #2154-100810-040814, #2264-101032-080315, and
#2273-101047-093015.
Funding Information Funding for this research was provided by the
Bonefish & Tarpon Trust.
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